1. Field of the Invention
The present invention relates to a developing device used in image forming apparatuses such as a copying machine and a printer for developing an electrostatic latent image formed on an electrostatic latent image carrier into a visible form.
In particular, the invention relates to an one-component developing device, in which toner supplied to a toner carrying member having a moving surface is passed between the surface and a toner restricting member contacting the above surface for forming a thin layer of charged toner held on the surface to be transferred and supplied to a developing region for the developing process, and remaining surplus toner which was not consumed in the developing process is returned to a toner supply side while being held on the toner carrying member.
2. Description of the Related Art
An example of such one-component developing device is schematically shown in FIG. 15.
The developing device shown in FIG. 15 includes a drive roller 91 which is driven by unillustrated drive means in the CCW direction in the figure. A flexible developing sleeve 92 is fitted around the drive roller 91. The developing sleeve 92 has an inner diameter slightly larger than an outer diameter of the drive roller 91. The sleeve is pressed against the drive roller 91 by a pressing guide 93 contacting rear sides of the opposite ends of the sleeve 92, and thereby a slackened portion 920 is formed at a position diametrally opposite to the pressed portions. The slackened portion 920 softly contacts an electrostatic latent image carrier (a photosensitive drum in this example) PC. A toner restricting blade 94 also contacts the developing sleeve 92 from the same side as the pressing guide 93.
At the rear of the developing sleeve 92, there is provided a buffer chamber 95, and a toner supply chamber 96 is provided at the rear of the buffer chamber 95. A toner supply rotary member 97 rotating in the CCW direction is arranged in the buffer chamber 95, and a toner agitating and supplying rotary member 98 rotating in the CW direction is arranged in the toner supply chamber 96.
A lower sealing member 99 is in contact with the lower surface of the developing sleeve 92 for preventing external leak of the toner from the buffer chamber 95.
In this developing device, the toner T, which was supplied from the toner supply chamber 96 into the buffer chamber 95 by the rotation of the rotary member 98, is sequentially supplied onto the surface of the developing sleeve 92 by the rotation of the toner supply rotary member 97.
Meanwhile, the toner T supplied to the sleeve 92, which is frictionally driven by the rotating drive roller 91, passes between the toner restricting blade 94 and the sleeve 92 and is thereby frictionally charged owing to the pressure by the blade 94. Also, the toner T, which takes a form of a thin layer of a predetermined thickness and is held on the sleeve surface, is transferred to the developing region confronting the photosensitive drum PC, and is used for developing the electrostatic latent image with a developing bias V.sub.B supplied by a power supply 921.
Surplus toner T after the development passes between the sealing member 99 and the sleeve 92 into the buffer chamber 95 in accordance with the rotation of the sleeve 92.
A major part of the toner returned into the buffer chamber 95 is separated from the sleeve 92, but a part of the highly charged toner remains on the surface of the sleeve 92. The remaining toner forms a micro electric field between it and the sleeve 92, and this electric field will attract the subsequently supplied toner onto the sleeve surface.
However, if the fluidity of the toner increases above the ordinary value, for example, under a low humidity environment, the chargeability of the toner also increases. This tends to increase the amount of the highly charged toner which is accumulated on the developing sleeve 92 without moving away from the same, and thus tends to increase the amount of the toner attracted to the developing sleeve 92. In this case, the restricting blade 94 cannot sufficiently restricts the amount of the toner adhered onto the sleeve 92, resulting in such disadvantage (so-called abnormal adhesion of toner) that an excessively large amount of the toner is transferred to the photosensitive drum PC and a non-image portion is also developed with the toner.
The restricting blade 94 repetitively applies the stress to the toner accumulated on the developing sleeve 92, so that the toner spreads and solidifies over the sleeve, which causes so-called filming of the toner, resulting in deterioration of the image quality. The stress described above is liable to deteriorate the toner (e.g., reduction of a diameter of toner particles and/or drop of fluidizing agent, i.e., silica), and thus may deteriorate a property such as followability to solid black.
Further, if the amount of the toner accumulated on the sleeve 92 increases, the toner newly supplied to the sleeve 92 is charged by the mutual friction of the toner in addition to the normal friction with the restricting blade 94, so that the amount of the toner having a polarity opposite to the regular charge polarity increases, resulting in deterioration of the image quality.
For the purposes of suppressing phenomena such as the abnormal adhesion of toner, filming of toner and deterioration of toner, the degree of charging by the friction may be reduced, for example, by reducing a width of mutually contacting planes of the toner restricting blade 94 and the sleeve 92 in order to adjust the amount of charge of the toner and to prevent the charge-up. In this case, the charge amount of the toner can be reduced and the micro-electric field formed between the toner and the sleeve 92 is weakened, so that separation of the toner from the sleeve 92 is promoted.
If the charge of the toner is reduced prior to the development of the electrostatic latent image, as described above, problems such as fog adhesion in the background and dispersion of toner around characters may generate after the development of solid black which is not subjected to the repetitive sliding by the restricting blade 94, because the charging ability of the restricting blade 94 has been reduced, in the case where the toner has the low chargeability due to the reduction of the fluidity, e.g., by the drop of fluidizing agent (silica) after the image forming operation was repeated many times, or in the case where the toner has a low chargeability due to a high humidity environment.
Further, in recent years, demands for reproduction of high resolution images have been increased. Since the quality of reproduced images depends on a particle diameter of toner, toner made of particles of a small diameter has been proposed. However, in the one-component developing device already described, remarkably presents in addition to the problem already described a problem that toner remaining on the sleeve 92 after the development, and particularly, toner formed of particles of a small diameter will not leave the sleeve 92 and be accumulated thereon, so that memory will generate on the sleeve and quality of the image will deteriorate after printing of many sheets.
Accordingly, it is an object of the invention to provide an one-component developing device for developing an electrostatic latent image on an electrostatic latent image carrier, wherein remaining surplus toner which was not consumed in the developing process can be easily separated from a toner carrying member even after an image forming operation was repeated many times, and thereby an amount of the toner transferred to a developing region and a charge of the toner are appropriately maintained to prevent problems such as accumulation of the toner on the toner carrying member, deterioration of the toner and filming of the toner, and thus, to obtain an image of a high quality without fog adhesion and dispersion.
Another object of the invention is to provide an one-component developing device for developing an electrostatic latent image on an electrostatic latent image carrier which can prevent disadvantageous accumulation of toner on a toner carrying member, filming of toner on the toner carrying member and deterioration of toner, and further, can prevent disadvantageous memory on the toner carrying member, which may be caused when using toner formed of particles of a small diameter, as well as deterioration of the image quality after printing of many sheets.